This invention relates generally to resistance-to-current converters, and more particularly to a converter which is responsive to the value of a sensing resistor such as a thermometer bulb that is subject to leakage resistance, the converter including means to cancel out the influence of such leakage on the output current signal.
When a two-wire transmission system is used for conveying a signal representing a change in a variable sensed by means of a resistance, such as a thermometer bulb responsive to changes in temperature, the existence of a leakage resistance gives rise to an error signal.
The conventional arrangement for this purpose is constituted by a bridge circuit, one of whose arms is formed by the sensing resistor, the output junctions of the bridge being connected to a converter that converts the voltage at these junctions to a corresponding current signal. This current signal is transmitted through a two-wire line to a load resistor at a remote station, one end of the resistor being grounded.
When a leakage resistance path is established between the sensing resistor and ground because of the degradation of the insulation of the sensing resistor or by reason of any other factor, an error signal is developed. As a consequence, the current signal produced across the load resistor does not accurately represent the variable being sensed, for it includes an error signal component.